If you decide you want to get your female a friend, you can go with some tetras or corys. However, if you want to get her a betta friend, you better get her at least two female betta friends. They tend to have a pecking order, and two can result in injury or death from fighting. Three will allow for a good pecking order. There will still be some initial flaring and possible nipping, but eventually, if their personalities allow for it, everything will settle down.